1. Technical Field
The present invention relates to wireless communications and, more particularly, wideband wireless communication systems.
2. Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards, including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, etc., communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels e.g., one of a plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via a public switch telephone network (PSTN), via the Internet, and/or via some other wide area network.
Each wireless communication device includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage (de-modulator). The low noise amplifier receives an inbound RF signal via the antenna and amplifies it. The one or more intermediate frequency stages mix the amplified RF signal with one or more local oscillations to convert the amplified RF signal into a baseband signal or an intermediate frequency (IF) signal. As used herein, the term “low IF” refers to both baseband and intermediate frequency signals. A filtering stage filters the low IF signals to attenuate unwanted out of band signals to produce a filtered signal. The data recovery stage recovers raw data from the filtered signal in accordance with the particular wireless communication standard. Alternate designs being pursued at this time further include direct conversion radios that produce a direct frequency conversion often in a plurality of mixing steps or stages.
Phase locked loops (PLLs) are becoming increasingly popular in integrated wireless transceivers as components for frequency generation and modulation. PLLs are typically used for one of a variety of functions, including frequency translation to up-convert a baseband (BB) frequency to an intermediate frequency (IF) or to one of a BB frequency or IF to RF prior to amplification by a power amplifier and transmission (propagation). PLLs allow for a high degree of integration and, when implemented with the appropriate amount of programmability, can form a main building block for modulators that operate over a wide range of frequencies. Typically, a baseband processor produces a baseband digital data that is converted to a continuous waveform signal by a digital-to-analog converter (DAC). The continuous waveform signal is a BB frequency signal that requires up-converting to IF and then RF.
A class of PLL based transmitters, known as translational loops, have become particularly popular. Briefly, in a translational loop, the desired modulated spectrum is generated as some low IF or baseband signal and then is translated to the desired RF using a PLL. In applications with non-constant envelope modulation, a parallel path for amplitude variation modulates the output power amplifier to generate the desired amplitude variation.
FIG. 1 illustrates a translational loop transmitter in accordance with some current designs for use in a global system for mobile communications (GSM) network. The example shows a so-called “quad” band transmitter, where four transmission bands are supported. Specifically, these bands are located in the 1900 MHz, 1800 MHz, 900 Mhz, and 800 MHz range. Generally, the transmitter of FIG. 1 includes a baseband processor that produces a low frequency digital signal that is converted by a DAC and is low-pass filtered to create a low frequency continuous waveform signal. A translational loop is then used to up-convert the low frequency continuous waveform signal to the desired transmission frequency for transmission from a power amplifier. Because this transmitter is utilized in a GSM network in which the information is conveyed in a phase-modulated carrier, the digital processor of the transmitter of FIG. 1 phase modulates the digital data.
More specifically, the transmitter of FIG. 1 includes a digital baseband processor, in-phase and quadrature digital-to-analog converters (DACs), corresponding low-pass re-construction filters, and analog baseband mixers. A summing node combines the mixer outputs, which are followed by low-pass filtering. The remaining components of the transmitter are a phase and frequency detector (PFD), a 26 MHz crystal reference, a charge pump, a loop low-pass filter (Loop Filter), a voltage controlled oscillator (VCO), a divide-by-2 module, a pair of offset mixers, as well as corresponding low-pass filters (LPFs). Radio frequency channel selection is achieved by employing a fractional-n (FRAC-N) frequency synthesizer.
A qualitative description of the operation of the translational loop is as follows. The sum of the mixing products of the baseband I & Q components with down-converted RF output I & Q components are low-pass filtered to generate a 26 MHz sinusoid whose excess phase component equals the difference between the desired baseband phase signal and the RF output phase signal. The 26 MHz IF is extracted by the PFD whose output is the phase error signal. As in any other properly designed PLL, the closed loop action of the loop causes the error signal to approach zero; hence, the phase of the RF output tracks the phase of the baseband signal, as desired.
FIG. 2A is a diagram that illustrates a power spectrum density (PSD) measured in decibels relative to the carrier (dBc) versus frequency measured in Mega-Hertz for the digital baseband processor of the radio transmitter of FIG. 1. As indicated, the sample rate of the digital processor for the GSM radio transmitter of FIG. 1 is 13 MHz. FIG. 2B illustrates the modulation error of a GSM translational loop transmitter as a function of baseband DC offset in % relative to full scale signal. FIG. 2C illustrates the modulation error of the example GSM translational loop transmitter as a function of baseband gain mismatch in % relative to full scale signal. Finally, FIG. 2D illustrates the modulation error of the example GSM translational loop transmitter as a function of RF phase imbalance in degrees.
The modulation performance of the translational loop architecture of FIG. 1 often degrades significantly in the presence of baseband DC offset as well as I/Q imbalances on the baseband side and the RF feedback path as may be seen in FIGS. 2A-2D. The negative impact of DC offset is particularly troublesome in low-voltage CMOS processes where it may constitute a significant fraction of the full-scale signal amplitude. In applications where relatively high modulation performance is required, such as in GSM cellular telephony, the modulation error introduced by DC offset and I/Q imbalances may exceed the permissible level even with application of careful analog design and calibration techniques intended to minimize these effects. For GSM cellular telephony, the root-mean-square (RMS) transmitter modulation error performance must be better than 5° and the peak modulation error must be better than 20°. It follows from FIGS. 2A-2D that for the transmitter of FIG. 1, baseband DC offsets should be limited to less-than 1% of the full scale signal, and the gain mismatch should be limited to less-than 3% for the transmitter to satisfy the modulation accuracy requirements. In particular, limiting DC offset to less than 1% of the full-scale signal is impractical or impossible in a low-voltage CMOS process, thus making it difficult to obtain these performance requirements.
Thus, a need exists for a modified translational loop RF transmitter architecture in which DC offset and I/Q imbalances do not pose limitations on transmitter modulation performance.